Cell growth both in plants and in animals, is orchestrated by a series of extracellular signals known as hormones or growth factors (Galinha et al. 2009, Semin Cell Dev Biol. 20: 1149-1156), which act through specific membrane receptors (Santner and Estelle 2009, Nature 459: 1071-1078; De, I et al. 2009, Nat Cell Biol 11: 1166-1173). These hormones can be synthesized within the plant or they can come from external organisms, as is the case of the factors produced by rhizobacteria which favor the growth of their symbiont plant (Lugtenberg and Kamilova 2009, Annu Rev Microbiol 63:541-556).
Five groups of growth factors in plants have been established: auxins, gibberellins, cytokinins, abscisic acid and its derivatives and ethylene. These substances are widely distributed and can, in fact, be found in all higher plants. They are specific in terms of their action, perform their activity at very low concentrations, and regulate cell growth, cell division and differentiation, as well as organogenesis, the senescence and the latency state. Less frequent though not entirely unknown, is the case in which a conservation of the structure of a growth factor occurs in such a way that said growth factor is functional both in plants and in animals. A typical example is the glycoprotein known as granulin, having representatives from fungi to mammals, and performing functions as varied as modulation of the vegetative growth in vegetables, or the regulation of cancer in animals (Bateman and Bennett 2009, Bioessays 31: 1245-1254).
For the purpose of regulating the development of plants and increasing plant biomass, a number of attempts to control the growth thereof by means of using chemical compounds have been made, such as, for example, that described in patent application EP0934951A1, or fertilizers. Patent application US2010/0016166A1 describes a method for increasing the number of seeds and flowers of a plant which comprises cultivating a plant in the presence of glutamate. International patent application WO2010/001184A1 describes a composition comprising (i) a micronized natural calcite mineral; (ii) micronized zeolite; and (iii) one or more additives for stimulating plant growth and improving the crop yield. However, mineral fertilization has a negative effect on agricultural production because the high concentrations of fertilizer can damage the soil and the desired results in terms of crop yield are not always obtained.
In recent years, and due to the advances in genetic manipulation, a new strategy has been developed for increasing the plant biomass consisting of controlling the expression of determined genes which control cell metabolism of the plant. In this sense, US patent application US2009/0094716A1 describes a method for increasing the plant biomass comprising the manipulation of the expression of the fve gene encoding a protein (FEV) having 6 copies of a WD40 domain. The inhibition of the expression of FVE provides the plant with an improved agricultural property, specifically, the increase in the yield of the biomass produced by the plant with respect to a control plant. In addition, international patent application WO2007/027866 describes the use of a recombinant DNA for the expression of proteins useful in controlling plant morphology, physiology and growth. Said recombinant DNA comprises a functional promoter in plants covalently bound to a nucleotide sequence encoding a protein which has at least one domain of the Pfam family of proteins. Patent application WO2009/003429A2 describes a method for regulating the biomass production in plants comprising the modification of the expression of the cki1 gene or of orthologs or homologs thereof. US patent application US2010/0037351A1 describes the increase of plant biomass, and with it the yield of the plant under hyperosmotic stress by means of the overexpression in the plants of the gene encoding the phospholipase Dε (PLDε). Nevertheless, the methods involving genetic manipulation of the plant are usually expensive and are not accepted by society.
International patent application WO2004/035798 discloses the identification of genes that are upregulated or downregulated in transgenic plants overexpressing E2Fa/DPa and the use thereof to alter plant characteristics; in particular, the protein of SEQ ID NO: 1848 is disclosed although its eventual use as plant growth factor is not shown.
Further, international patent application WO2004/035798 discloses, in general, a method for producing a plant with increased yield as compared to a corresponding wild type plant comprising at least increasing the activities of a group of proteins, including the proteins of SEQ ID NO: 4659 and 4660, although the ability of said proteins to increase plant biomass is not disclosed.
Therefore, there is a need in the state of the art to develop alternative methods to those already existing for increasing the plant biomass and with it, the crop yield, which do not have the aforementioned drawbacks.
It has now been found that the administration of adrenomedullin to plants and algae increases their biomass. Adrenomedullin (AM) and the peptide of 20 amino acids of the N-terminal region of proadrenomedullin (PAMP) come from post-translational processing of the same protein, preproadrenomedullin, which is encoded by the adm gene. Despite not having any structural similarity, both peptides produce similar and a large number of physiological responses. Among these are their vasodilator effect, bronchodilator effect, cell motility and growth regulator effect, modulator effect for the secretion of other hormones and intestinal absorption regulatory effect (López, J and Martínez, A. 2002. Int Rev Cytol 221: 1-92). In the context of cancer cells, AM acts like a growth factor, promotes cell motility, reduces apoptosis and induces angiogenesis (Martínez et al. 2002, J Natl Cancer Inst 94: 1226-1237).